LIDAR (Light Detection and Ranging) devices are widely used in numerous detection and measurement applications. Such applications include wind speed measurement, fluid flow analyses, the study of vibration phenomena, vehicle speed measurement, applications in the field of air transport, etc. Their operation consists in emitting a coherent optical wave in the direction of a target volume where an object or a phenomenon to be studied is located, and to collect a portion of this wave which is backscattered from the target volume. In a coherent LIDAR device, the collected portion of the scattered wave undergoes heterodyne detection. The speed measurement is determined from a Doppler shift which is measured in a signal of the heterodyne detection.
However, the scattered wave which is collected may originate not only from the object in the target volume which the LIDAR device is pointing towards, but also from objects outside the target volume which may also significantly scatter the optical wave. For example, for a wind speed measurement, the wave is scattered by the droplets or dust within the target volume which are carried along by the wind, but a cloud which is located beyond the target volume can cause much more optical scattering than the droplets and dust in the target volume. In this case, the presence of the cloud in the background, and possibly the motion of the cloud, can interfere with or prevent the detection of the portion of the wave which is scattered by the content of the target volume. A comparable situation occurs when measuring the flow of a fluid, and a wall in the background produces significant scattering or reflection of the wave emitted by the LIDAR device.
In LIDAR devices where the system for emitting the optical wave is separate from the system for receiving the backscattered wave, the target volume can be selected relative to the background. The two emitting and receiving systems are each oriented towards the target volume, with the target volume selected relative to the background at the intersection of the respective directions in which the emitting and receiving systems are pointing. Such LIDAR devices, known as bistatic LIDAR, require precise installation in order to select the desired target volume, with a sufficient distance separating the emitting and receiving systems. They are therefore poorly suited for use on board a vehicle or an aircraft, for example. Usually they are difficult to move because of their structure involving several separate parts. Bistatic LIDAR devices are also inappropriate for mapping speed measurements by varying the target volume within a much larger volume of analysis.
Monostatic LIDAR devices do not have these disadvantages, as they combine the wave emitting and receiving systems. They can easily be moved and used in vehicles and aircrafts. However, speed measurements obtained with such monostatic devices can be impeded and prevented by a backscattering element located in the direction in which the device is pointing, in front of or behind the target volume, even if the distance from the target volume is taken into account in the heterodyne detection.
In addition, a monostatic LIDAR device allows easily mapping speed measurements, by varying the direction the device is pointing in order to scan a field to be analyzed.
To select the useful portion of the heterodyne detection signal which contains the measurement information, and not the part of the same heterodyne detection signal which does not originate from the object of interest, it is known to repeat the acquisition of the heterodyne signal, separately analyze the heterodyne detection signals obtained at each repetition, and accumulate all the results from these analyses. Such accumulation usually consists in adding the results from the spectral analyses. This allows separating the measurement information concerning the object of interest when this information is constant, from the variable or incoherent background noise. Such processing does not distinguish between two objects situated in the pointing direction of the monostatic LIDAR, and which each have a substantially constant moving speed.
It is also known to perform a synchronous detection of the wave portion which is scattered, to distinguish it from contributions to the detected signal which are independent of the optical wave emitted.
Under these conditions, a first object of the invention is to provide speed measurements using a monostatic LIDAR device with heterodyne detection, which does not have the above disadvantages.
A second object of the invention is that such measurements can be performed using LIDAR devices existing before the invention, by easily and only slightly modifying such devices and their use.
In particular, the invention has the object of measuring speeds with an improved selectivity of the target volume relative to the foreground and background in the pointing direction of the device. Such improved selectivity is desirable for mapping measurements.
Another object of the invention is to enable measurements using a LIDAR device which takes up little space, and for which the weight is not significantly increased relative to existing devices.